1. Field of the Invention
The invention relates to an inter-cylinder air/fuel ratio imbalance determination apparatus and an inter-cylinder air/fuel ratio imbalance determination method.
2. Description of Related Art
An air/fuel ratio control apparatus as shown in FIG. 1 that includes a three-way catalyst 43 disposed in an exhaust passageway of a multi-cylinder internal combustion engine 10, and an upstream-side air/fuel ratio sensor 56 disposed upstream of the three-way catalyst 43 has been widely known.
This air/fuel ratio control apparatus calculates an air/fuel ratio feedback amount (main feedback amount) on the basis of the output value of the upstream-side air/fuel sensor 56 and performs a feedback control of the air/fuel ratio of an engine 10 by the feedback amount so that the air/fuel ratio of a mixture supplied into the engine 10 (the air/fuel ratio of the engine and, therefore, the air/fuel ratio of exhaust gas) becomes equal to a target air/fuel ratio. This feedback amount is a control amount that is common to all the cylinders. The target air/fuel ratio is set at a predetermined reference air/fuel ratio within a window of the three-way catalyst 43. The reference air/fuel ratio is generally the stoichiometric air/fuel ratio. The reference air/fuel ratio can be altered to a value in the vicinity of the stoichiometric air/fuel ratio according to the amount of air taken into the engine and the degree of degradation of the three-way catalyst 43.
Incidentally, the air/fuel ratio control apparatus as described above is generally applied to an internal combustion engine that adopts an electronically controlled fuel injection apparatus. In such an internal combustion engine, at least one fuel injection valve 33 is provided for each cylinder of each of the intake ports that communicate with the cylinders. Therefore, if the characteristic of the fuel injection valve of a specific cylinder becomes a “characteristic of injecting an excessive amount of fuel that is greater than a commanded amount of fuel injection (commanded fuel injection amount)”, only the air/fuel ratio of mixture supplied to that specific cylinder (the air/fuel ratio of that specific cylinder) changes to the rich side. That is, the non-uniformity in the air/fuel ratio among the cylinders (variations in the air/fuel ratio among the cylinders, the inter-cylinder imbalance proportion regarding the air/fuel ratio) becomes large. In other words, there occurs conspicuous imbalance among the “cylinder-by-cylinder air/fuel ratios” that are the air/fuel ratios of the mixture supplied into the individual cylinders, and the degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios becomes large.
Incidentally, in the following description, the cylinder that corresponds to a “fuel injection valve that has a characteristic of injecting an amount of fuel that is excessively larger or excessively smaller than the commanded fuel injection amount” is also referred to as “imbalance cylinder”, and the other cylinders (the cylinders that correspond to the fuel injection valves that inject the commanded fuel injection amount of fuel“) are also referred to as “none-imbalance cylinder (or normal cylinders)”.
If the characteristic of the fuel injection valve of a specific cylinder becomes a characteristic of injecting an amount of fuel that is excessively larger than the commanded fuel injection amount”, the average of the air/fuel ratios of the mixture supplied into the engine as a whole becomes an air/fuel ratio on the rich side of the target air/fuel ratio that is set at the reference air/fuel ratio. Therefore, due to the feedback amount of the air/fuel ratio that is common to all the cylinders, the air/fuel ratio of the aforementioned specific cylinder is changed to the lean side so as to approach the reference air/fuel ratio, and simultaneously, the air/fuel ratio of the other cylinders is changed to the lean side so as to move away from the reference air/fuel ratio. As a result, the average of the air/fuel ratios of mixture supplied to the engine as a whole (the average air/fuel ratio of exhaust gas) equals an air/fuel ratio in the vicinity of the reference air/fuel ratio.
However, the air/fuel ratio of the aforementioned specific cylinder is still an air/fuel ratio on the rich side of the reference air/fuel ratio, and the air/fuel ratio of the other cylinders is an air/fuel ratio on the lean side of the reference air/fuel ratio. As a result, the amount of emission discharged from each cylinder (the amount of unburned material and/or the amount of nitrogen oxides) increases, in comparison with the case where the air/fuel ratio of each cylinder is equal to the reference air/fuel ratio. Therefore, even if the average of the air/fuel ratios of mixture supplied to the engine as a whole is equal to the reference air/fuel ratio, the increased amount of emission cannot be purified by the three-way catalyst, so that a possibility of deterioration of the emission arises.
Hence, in order to avoid deterioration of the emission, it is important to detect excessively large non-uniformity in the air/fuel ratio among the cylinders (excessively large non-uniformity in the air/fuel ratio among the cylinders, that is, occurrence of the inter-cylinder air/fuel ratio imbalance state) and take some countermeasures. Incidentally, the inter-cylinder air/fuel ratio imbalance also occurs in, among others, the case where the characteristic of the fuel injection valve of a specific cylinder has become a “characteristic of injecting an amount of fuel that is excessively smaller than the commanded fuel injection amount”.
A related-art inter-cylinder air/fuel ratio imbalance determination apparatus acquires a value of the locus length of an output value (output signal) of an electromotive force type oxygen concentration sensor disposed upstream of the three-way catalyst 43 as an “air/fuel ratio imbalance index value (imbalance determination-purpose parameter)”. Furthermore, this determination apparatus compares the locus length and a “reference value that changes according to the engine rotation speed” and, on the basis of a result of comparison, determines whether or not the inter-cylinder air/fuel ratio imbalance state has occurred (see, e.g., U.S. Pat. No. 7,152,594). The determination as to whether or not the inter-cylinder air/fuel ratio imbalance state has occurred is also referred to simply as “imbalance determination”.
The air/fuel ratio imbalance index value that makes it possible to determine whether or not the inter-cylinder air/fuel ratio imbalance state is occurring by comparing the index value with the imbalance determination threshold value is a parameter that increases with increases in “the degree of non-uniformity in the cylinder-by-cylinder air/fuel ratio between a plurality of cylinders (degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios).
On the other hand, one of the related-art air/fuel ratio control apparatuses adopts a so-called “limiting current type air/fuel ratio sensor” as the upstream-side air/fuel ratio sensor 56. In this construction, the air/fuel ratio imbalance index value is acquired as an air/fuel ratio fluctuation index quantity that becomes greater the greater the fluctuation of the output value of the upstream-side air/fuel ratio sensor. This is because if the degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios becomes great, the exhaust gases from imbalance cylinders and the exhaust gas from the non-imbalance cylinders are sequentially discharged, so that the greater the degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios becomes, the greater the fluctuation of the air/fuel ratio of exhaust gas becomes. Incidentally, in the foregoing description, the limiting current type air/fuel ratio sensor is also referred to simply as “air/fuel ratio sensor”.
The air/fuel ratio fluctuation index quantity can be acquired on the basis of “various basic index quantities calculated on the basis of the output value of the air/fuel sensor” as described below. Representative examples of the basic index quantities include time-regarding “differential values (a time differential value, that is, a slope), and the second-order differential value, etc., such as “an output value of the air/fuel ratio, a high-pass filter-processed output value obtained through the high-pass filter processing of the output value of the air/fuel sensor, and the air/fuel ratio represented by the output value of the air/fuel ratio (upstream-side air/fuel ratio)”, etc.
However, the response of the limiting current type air/fuel ratio sensor (the change in the output value of the air/fuel sensor relative to the change in the air/fuel ratio of exhaust gas to be detected) differs among individual air/fuel sensors. That is, the air/fuel ratio sensors have individual product differences. Therefore, in the case where the degree of non-uniformity in the cylinder-by-cylinder air/fuel ratio is “a specific value”, the output value of a high-response air/fuel ratio sensor fluctuates as shown in FIG. 10A, and the output value of an air/fuel ratio sensor that has a responsiveness equal to a center of the tolerance fluctuates as shown in FIG. 10B, and the output value of a low-response air/fuel ratio sensor fluctuates as shown in FIG. 10C. That is, even if the degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios is a “specific value”, the manner of the fluctuation of the output value of an air/fuel ratio sensor varies depending on the responsiveness of the air/fuel ratio. Therefore, even if the air/fuel ratio fluctuation index quantity is fixed at a “certain value”, there occurs a case where the degree of non-uniformity of the cylinder-by-cylinder air/fuel ratios varies. As a result, if the imbalance determination is executed on the basis of comparison between the air/fuel ratio imbalance index value obtained on the basis of the air/fuel ratio fluctuation index quantity and the imbalance determination threshold value, there is a possibility of false determination.